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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP Understanding RSTP
To be consistent with Cisco STP implementations, this guide documents the port state as blocking
instead of discarding. Designated ports start in the listening state.

Rapid Convergence

The RSTP provides for rapid recovery of connectivity following the failure of switch, a switch port, or
a LAN . It provi des r ap id c onvergence for edg e por ts, new root po rts, and por ts c onn ect ed t h rough
point-to-point links as follows:
•Edge ports—If you configure a port as an edge port on an RSTP switch by using the spanning-tree
portfast interface co nfiguration command, the edge port immediately tra n sitions to the forwarding
state. An e dg e por t is t he sam e as a Po rt Fast -ena bled p ort, a nd you sh ould e nab le it on ly on port s
that connect to a single end station.
•Root ports—If the R STP sele cts a new root p ort, it bl ock s th e old roo t po rt and im med iat ely
transit ions the new root port t o the forwar ding state .
•Point-to-point links—If you connect a port to another port through a point-to-point link and the local
port bec om es a de signa ted po rt, it n egotiat es a ra pid t ransit ion with t he o ther p ort by usi ng the
propos al-agr eement ha ndshake to ensure a loop- free topology.
As shown in Figure 12-1, Switch A is connected to Switch B through a point-to-point link, and all
of the p orts ar e in th e bl ocki ng state. Assume that the priority of Switch A is a smaller numerical
value than the prio rity of Switc h B. Switch A sends a pr oposal me ssage (a configura tion brid ge
protocol data unit [BPDU ] with the p roposal flag se t) to Switch B, proposing itself as the designated
switch.
After re ceiving the proposal message, Swi t ch B selects as its new root port the port from w hich the
propos al m essag e was re ceived, forc es all non ed ge po rt s to the b loc king stat e, an d s en ds an
agreement message (a BPDU with the agreement flag set) through its new root port.
After receiving Switch B’s agreement message, Switch A also immediately transitions its designated
port to the forwarding sta te. No loops in the netw ork ar e for med be caus e Switch B block ed all of its
noned ge po rts an d b eca use t here is a po in t-to-p oi nt li nk b etwee n Switc hes A an d B.
When S witc h C is co nnect ed to Swi tch B, a s imilar set of han dsha kin g mess ages ar e exchan ged.
Switch C select s the port conn ected to Switch B as i ts root port, and both ends immed iately
transition to th e f orw ar din g state. With each iteratio n of this handsh akin g pr ocess, o ne m ore sw itch
joins the a ctive topology. As the network converges, this pro posal-agre ement handshaki ng
progresse s fro m th e root toward the le aves of the spanni ng tree .
The switch determines the link type from the port duplex mode: a full-duplex port is considered to
hav e a point-to-point connection; a half-duplex port is considered to have a shared connection. You
can overrid e th e def a ult setting that is de termined by the dupl e x settin g by using the spanning-tree
link-type interface co nfiguration c omman d.
Enabled Forwarding Forwarding Yes
Disabled Disabled Discarding No
Table 12-1 Port State Comparison (continued)
Operatio nal Status STP Port Sta t e RSTP Port State Is Port Included in th e
Active Topology?